1. Field of the Invention
The present invention relates to a failure diagnostic device for an exhaust gas recirculation (EGR) system, which determines an occurrence of a failure based on a differential pressure of intake air pressures in an intake pipe disposed downstream of a throttle valve between a passage-opening control and a passage-closing control of a recirculation pipe through which exhaust gases are returned to the intake pipe.
2. Description of the Related Art
Hitherto, EGR systems have been widely used in an internal combustion engine for reducing NOx (nitrogen oxides) emissions in exhaust gases. In those types of the EGR systems, a portion of the exhaust gas is returned to an intake pipe through a recirculation pipe, whereby inert gas components in an air-fuel mixture are increased to lower a combustion temperature. Accordingly, if there occurs such a trouble as a blockage of the recirculation pipe or an operation failure of an EGR control valve disposed in the recirculation pipe, a problem arises in, for example, that an air-fuel ratio becomes abnormal. As a result, a combustion is brought into an unstable state and a degradation of exhaust gas emissions is caused.
As one of techniques for detecting a failure of the EGR system, Japanese Unexamined Patent Application Publication No. 8-82253, for example, discloses a technique of detecting a pressure change in an intake pipe downstream of the throttle valve, thereby determining whether or not the EGR system is failed. More specifically, according to the disclosed technique, when diagnosis run (or executing) conditions are satisfied, the intake pipe pressure during an open state of an EGR valve is detected plural times. Then, the EGR valve is closed and the intake pipe pressure during the closed state of the EGR valve is detected for plural times. A diagnosis value is computed from an average value of the intake pipe pressures during the open state of the EGR valve and the average value of the intake pipe pressures during the closed state of the EGR valve. Then, a failure of the EGR system is determined if a computed diagnosis value is deviated from a value for the determination of a normal state. In such a process, to prevent false diagnosis of the EGR system with a higher reliability, the diagnosis is ceased (cancelled) and the detected values are discarded, for example, when such operation conditions as a throttle opening degree and an engine speed are changed in excess of respective predetermined thresholds, or when the difference between an atmospheric pressure and each detected intake pipe pressure is not larger than a predetermined value.
In the above-described technique, however, to cancel the diagnosis as appropriate for a purpose of improving a measurement accuracy of the intake pipe pressure, various cancel conditions must be individually determined in specific manners. Therefore, the process for the failure diagnosis is more complicated.
Also, when control errors are caused in various kinds of engine controls and so on due to, e.g., a failure of any other components than the EGR system, there is a risk that the diagnosis cannot be properly cancelled. Such a case may lead to a risk that the detected value of the intake pipe pressure becomes an improper value and a false diagnosis result is determined.
In view of the state of the art set forth above, it is an object of the present invention to provide a failure diagnostic device for the EGR apparatus, which can detect the intake pipe pressure at a high accuracy with a simple control and can prevent the false diagnosis of the EGR system.